It's usually a costs vs. benefits decision.
Costs:
- Create your own CA infrastructure or buy a public certificate for each sender
- Teach employees how to use it
- Teach employees how not to use it, especially how to make sure that the secret key is really kept secret
- Teach the customers what this strange stuff in the mail means
- Properly deal with certificate expiration, revocation and all this stuff
- ...
Benefits:
- Usually the argumentation goes like this: nobody else is using this so there cannot be lots of benefits
Thus unless the benefits are higher than the costs or some regulations require the use of signed mails it will not be implemented.